The phenolic resin provides a molding excellent in mechanical properties, and hence has widely been employed from old either independently or in the form of a blend with another resin, such as an epoxy resin. However, the phenolic resin per se and blend have drawbacks in that the light and alkali resistance is relatively low, that they are likely to absorb water or an alcohol to thereby suffer from changes in the dimension and electrical resistance, and that the thermal resistance, especially the oxidation resistance at high temperatures, thereof is poor.
In order to overcome the drawbacks, various modifications of the phenolic resin have been studied. For example, various modified phenolic resins have been proposed, which have improved resistance to deterioration and oxidation due to light, chemicals, etc. by virtue of the modification using a fat, an oil, a rosin or a neutral aromatic compound.
Among the various proposals, Japanese Patent Laid-Open Publication No. 61(1986)-235413 discloses a phenolic resin having excellent thermal resistance, obtained by selecting reactants of a phenol-modified aromatic hydrocarbon resin. However, the phenolic resin obtained by this method is disadvantageously not cured unless being maintained at a high temperature for a prolonged period of time in the manufacturing of a molding by the use thereof.
Japanese Patent Laid-Open Publication No. 2(1990)-274714 discloses that a modified phenolic resin useful for a molding material, having excellent thermal and oxidation resistance and mechanical strength as cannot be expected from the conventional phenolic resin, is obtained by employing a petroleum heavy oil or pitch, which is a cheap material, as a modifier material and by selecting specific reaction conditions.
Further, Japanese Patent Laid-Open Publication No. 4(1992)-145116 discloses that, in the production of such a phenolic resin, a crude modified phenolic resin obtained by a polycondensation of starting compounds is subjected to a neutralization treatment, a water washing treatment and/or an extraction treatment to thereby neutralize and remove any acid remaining in the crude modified phenolic resin, so that a modified phenolic resin which does not corrode a metal member brought into contact with the resin is provided.
In the above process for producing the modified phenolic resin, the acid remaining in the crude modified phenolic resin is actually neutralized and removed by the neutralization treatment using an amine, followed by the water washing treatment. However, the modified phenolic resin obtained through the purification step comprising the above neutralization and water washing treatments is likely to retain a neutralization product therein, so that there is a problem that it is unsatisfactory as a molding material used for a product on which strict requirements for thermal and corrosion resistance are imposed, such as a molding material for electrical or electronic part and a material for semiconductor sealer.
Japanese Patent Application No. 5(1993)-40646 teaches that a modified phenolic resin containing substantially no acid can be obtained by purifying a crude modified phenolic resin through a purification step including a specific extraction treatment. The modified phenolic resin containing substantially no acid, obtained through this purification step, may be combined with an epoxy resin, so that a molding material can be obtained which not only has excellent thermal and moisture resistance but also does not corrode any metals.
However, the above modified phenolic resin has a drawback in that the melt viscosity of the resin is so high that the resin is not suitable for speedy mass production of a molded article having a complex configuration. In addition, further improvements of thermal resistance, dimensional stability and strength and other mechanical properties have been demanded in the use of the modified phenolic resin in combination with an epoxy resin.
The inventors have made extensive and intensive studies with respect to the above drawbacks of the prior art. As a result, it has been found that a novel modified phenolic resin having a low resin melt viscosity and markedly improved in the reactivity with an epoxy resin can be provided by reacting the modified phenolic resin with a phenol in the presence of an acid catalyst to thereby lower the molecular weight of the modified phenolic resin.
Moreover, the inventors have found that a modified phenolic resin having a low resin melt viscosity, exhibiting high reactivity with an epoxy resin and containing substantially no acid can be provided by purifying the modified phenolic resin with an extraction solvent and/or water washing to thereby remove the acid catalyst, etc. contained in the modified phenolic resin. The present invention has been completed based on these findings.